The optical detection of small defects such as pinholes in continuously-produced sheets of materials such as aluminum foil, polymer films or paper is an important requirement for many materials processing industries.
In some cases, such as in the production of sealed metallic foils for food containers or in the production of plastic films for electric-insulation applications, the product must be guaranteed defect-free so that a 100% inspection is required. In other cases, an automated sampling procedure may be applied to determine the trends in the average density of pinholes across the product for statistical quality control and process monitoring requirements.
Pinholes in insulating materials are sometimes detected by electric-conductance devices using high-voltage brushes or sponges in contact with both sides of the sheet for pinhole detection through the establishment of spark discharges between the electrodes. Such techniques are often unreliable, low-speed, and subject to wear and erosion problems. Another approach is by liquid or gaseous leak testing on the assembled container. This approach is quite expensive in its implementation and in any case cannot be used by the sheet-producing company which must guarantee a pinhole-free sheet product to the container-manufacturing company.
Optical inspection techniques have been increasingly used for these applications in the last years. Optical methods are attractive because they are noncontact and thus easy to implement and rapid to scan over large sheets moving at high speed. A typical known apparatus for the optical detection of relatively large pinholes comprises a video camera used to image the moving sheet which is to be inspected. Backside flash illumination may be used to localize the pinhole position in fast-moving sheets with good spatial resolution. The camera may alternatively be situated on the same side as the illumination source to detect defects which do not correspond to a perforation of the sheet.
This known approach is mainly useful for the detection of relatively large pinholes, of the order of 1 mm.sup.2 in size. For the detection of very small pinholes, of the order of 10 .mu.m in diameter, the camera must be equipped with a close-up lens of the microscope kind. Such objectives have a typical operating distance of a few mm and a numerical aperture (N.A.) of the order of 0.5 to resolve a pinhole diameter d of the order of 10 .mu.m. This leads to a reduced field of view of less than 1 cm.sup.2 and to a depth of field of the order of d/NA.perspectiveto.20 .mu.m, hardly compatible with typical industrial requirements where a 1 meter-wide sheet is being drawn at speeds of 10 m/s with transverse fluctuations of several mm of amplitude.
Another known technique includes the reduction of the spatial resolution requirements to extend the field of view and the depth of field. A standard camera objective is used to image an area of typically 1 m.sup.2 with a 1 mm spatial resolution. This results in a depth of field of several cm, relaxing positioning requirements. However, defects smaller than 1 mm in size often escape detection unless a very strong illumination power is used to compensate for the low pinhole/pixel surface ratio.
Still another known technique includes spatial filtering under continuous illumination. A high-power continuous source is used for backside illumination of the moving sheet. The pinhole imaged during the camera integration time of 1/30 of a second will appear as a short line in the camera image. A sheet moving at 10 m/s will displace through 0.3 m during the camera integration time. Knowing the direction of the sheet movement, the camera image may be digitally filtered to enhance the visibility of lines oriented along such a direction. Again, spatial resolution considerations make this approach not sensitive to very small defects.
Concerning optical reflective techniques, there are two basic methods for reflective optical inspection systems: camera viewing under incoherent illumination, (using lamps, as in U.S. Pat. No. 4,162,126), or laser scanning (most often with rotating mirrors such as in U.S. Pat. No. 4,632,546). Incoherent illumination avoids speckle but it is affected by a number of problems including reduced illumination power density, limited depth of field, as well as long scanning time and difficulty to keep a convenient air purge over a large window aperture when a two-dimensional matrix-array camera is used. Laser scanning offers high instantaneous power, strong immunity to ambient light, long depth of field and convenient air purging through a slit, but it requires a delicate mechanical scanning device subject to long-term wear, it is subject to speckle noise, and requires a very high speed detector to resolve each pixel during a line scan.
It is an object of the present invention to provide apparatus and method for detecting the presence of flaws smaller than the flaws detected with known apparatuses and methods.